Centroamerica is never cheap, and unlike the usual airline price model, the further you travel from US, the more expensive it is.

Travel within Centroamerica tends to be even worse pricewise, though I have no idea what it would be from SJU. I do know prices TO SJU tend to be rather cheap because of lots of competition.

Wait, you do mean SJU, not SJO, right, because SJO tends to run in $600 range, while SJU tends to be much cheaper.

Long, long time ago TACA used to have Christmas Day / New Year Day sale when you could pick up a ticket very, very cheap, but I doubt they do that anymore.